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I have a dataset that merges very nicely in C2 and I222 (Rmerges ~5% and ~8% resp), 100% complete to 1.75A. The protein is a TIM-barrel enzyme, a couple of loops are expected to be in different orientations or missing with respect to the search model. The diffraction is very clean and appears to represent a single lattice. The crystal is from a cluster with the same symmetry but which presented multiple lattices upon diffraction...I just recently managed to separate a single crystal and was very excited when I saw the diffraction at the synchrotron.... The search model and target sequence have 75% identity and so I was expecting this to be a routine MR problem. I have mutated the sequence of the search model anyway to match the target sequence (I got tired of mutating the 25% non-identical whilst fitting MR solutions..). Two loops are deleted (~30 resdiues). I find oustanding solutions easily in both spacegroups (no solutions in I212121), using a number of programs including MOLREP, PHASER and CNS. None of these refine (REFMAC5 and CNS). For the best solutions I have fit the model to the density and the refinement does not improve (currently stuck at Rfree ~44%, Rfactor 40%). There are no obvious large errors excepting some missing loops residues (~20 still). The best solution (MOLREP) shows some difference density for one of the missing loops and waters and overall the maps are good. I have checked for merohedral twinning (C2) and pseudo-translational NCS; neither are present. The indexing output from MOSFLM: PENALTY 7 158 mP 55.55 82.57 82.27 90.1 109.9 109.5 P2,P21 6 12 oI 55.55 102.35 116.68 89.6 89.8 90.0 I222,I212121 5 10 mC 116.44 116.68 55.55 89.8 118.5 89.8 C2 4 8 mC 129.04 102.35 55.55 90.0 115.3 90.3 C2 3 5 mC 154.73 55.55 102.35 90.0 131.1 89.8 C2 2 2 aP 55.55 82.27 82.57 90.1 109.5 109.9 P1 1 0 aP 55.55 82.13 82.57 76.8 70.5 70.4 P1 I have tried solutions 6 and 4 (solution 4 is found by HKL2000 and I tried this first after I222). I have still to try 3 and 5 (3 looks non-standard with the large beta angle though, ..), which I just noticed when I re-indexed using MOSFLM today...at least with a similar crystal I have tried P1 and didn't get an MR solution. In lieu of running another hundred MR jobs with multiple spacegroups I was wondering if anyone had encountered anything like this before ?. Is there something about C2 I am discounting ? I was expecting this problem to disappear, having separated a single crystal from the cluster and therefore removed the additional lattices from the diffraction pattern, but this is apparently not the problem... Thank you very much in advance for any help. Cheers, Iain
